How does one write a long vowel in katakana?

I am curious about how certain long vowel sounds are written in katakana, e.g. my name, Karl would be "kaaru," but I don't know if that would be written In Karate class, my Japanese teacher wrote it as カール, or カアル。 How does one write "Makudonarudo hamba-ga-?" :)

Konnichiwa Karl or カールさん。 Thanks for your question! Arigato! Well, Makudonarudo hamba-ga- is written in the following way: マクドナルド　ハンバーガー Anytime there is a long vowel sound, such as your name Karl, in katakana it is shown by a kind of dash called a “choonâ€